Abstract
Mitogen-activated protein kinase (MAPK) cascades are important signal transduction pathway in plant immunity. SIPK and WIPK play pivotal roles in tobacco plants. To investigate downstream signaling of StMPK1, we identified several PPSs (protein phosphorylated by StMPK1) using in vitro expression cloning method. To dissect the biological function of PPSs in plant immunity, we employed virus-induced gene silencing (VIGS) in Nicotiana benthamiana. VIGS of NbWRKY4 (NbPPS8), enhanced disease susceptibility to a virulent strain of Phytophthora infestans. Recombinant NbWRKY4 was phosphorylated by StMPK1 and StWIPK in vitro. In addition, StMPK1 or StWIPK mediated phosphorylation of NbWRKY4 promoted its W-box sequence, cognate binding sequence for WRKY-type transcription factor, binding activity. Furthermore, NbWRKY4 expression was induced by the overexpression of NtMEK2DD, constitutive active mutant of NtMEK2. These results suggest that NbWRKY4 is regulated at the transcriptional and post-transcriptional levels by MAPKs and, involved in the plant immune responses.
